Add --synchronous option to pg_receivexlog, for more reliable WAL writing.

Previously pg_receivexlog flushed WAL data only when WAL file was switched.
Then 3dad73e7 added -F option to pg_receivexlog so that users could control
how frequently sync commands were issued to WAL files. It also allowed users
to make pg_receivexlog flush WAL data immediately after writing by
specifying 0 in -F option. However feedback messages were not sent back
immediately even after a flush location was updated. So even if WAL data
was flushed in real time, the server could not see that for a while.

This commit removes -F option from and adds --synchronous to pg_receivexlog.
If --synchronous is specified, like the standby's wal receiver, pg_receivexlog
flushes WAL data as soon as there is WAL data which has not been flushed yet.
Then it sends back the feedback message identifying the latest flush location
to the server. This option is useful to make pg_receivexlog behave as sync
standby by using replication slot, for example.

Original patch by Furuya Osamu, heavily rewritten by me.
Reviewed by Heikki Linnakangas, Alvaro Herrera and Sawada Masahiko.

doc/src/sgml/ref/pg_receivexlog.sgml

@@ -48,6 +48,13 @@ PostgreSQL documentation
     <application>pg_receivexlog</application>.
   </para>
 
+  <para>
+   Unlike the standby's WAL receiver, <application>pg_receivexlog</>
+   flushes WAL data only when WAL file is closed, by default.
+   <literal>--synchronous</> option must be specified to flush WAL data
+   in real time and ensure it's safely flushed to disk.
+  </para>
+
   <para>
    The transaction log is streamed over a regular
    <productname>PostgreSQL</productname> connection, and uses the replication
@@ -85,21 +92,6 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
-     <varlistentry>
-       <term><option>-F <replaceable class="parameter">interval</replaceable></option></term>
-       <term><option>--fsync-interval=<replaceable class="parameter">interval</replaceable></option></term>
-       <listitem>
-        <para>
-        Specifies the maximum time to issue sync commands to ensure the
-        received WAL file is safely flushed to disk, in seconds. The default
-        value is zero, which disables issuing fsyncs except when WAL file is
-        closed. If <literal>-1</literal> is specified, WAL file is flushed as
-        soon as possible, that is, as soon as there are WAL data which has
-        not been flushed yet.
-        </para>
-       </listitem>
-      </varlistentry>
-
      <varlistentry>
       <term><option>-n</option></term>
       <term><option>--no-loop</option></term>
@@ -135,15 +127,25 @@ PostgreSQL documentation
          When this option is used, <application>pg_receivexlog</> will report
          a flush position to the server, indicating when each segment has been
          synchronized to disk so that the server can remove that segment if it
-         is not otherwise needed.  When using this parameter, it is important
-         to make sure that <application>pg_receivexlog</> cannot become the
-         synchronous standby through an incautious setting of
-         <xref linkend="guc-synchronous-standby-names">; it does not flush
-         data frequently enough for this to work correctly.
+         is not otherwise needed. <literal>--synchronous</literal> option must
+         be specified when making <application>pg_receivexlog</> run as
+         synchronous standby by using replication slot. Otherwise WAL data
+         cannot be flushed frequently enough for this to work correctly.
         </para>
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--synchronous</option></term>
+      <listitem>
+       <para>
+        Issue sync commands as soon as there is WAL data which has not been
+        flushed yet. Also status packets are sent back to the server just after
+        WAL data is flushed whatever <literal>--status-interval</> is set to.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-v</option></term>
       <term><option>--verbose</option></term>

src/bin/pg_basebackup/pg_basebackup.c

@@ -370,7 +370,7 @@ LogStreamerMain(logstreamer_param *param)
 	if (!ReceiveXlogStream(param->bgconn, param->startptr, param->timeline,
 						   param->sysidentifier, param->xlogdir,
 						   reached_end_position, standby_message_timeout,
-						   NULL, 0))
+						   NULL, false))
 
 		/*
 		 * Any errors will already have been reported in the function process,

src/bin/pg_basebackup/pg_receivexlog.c

@@ -36,10 +36,10 @@ static char *basedir = NULL;
 static int	verbose = 0;
 static int	noloop = 0;
 static int	standby_message_timeout = 10 * 1000;		/* 10 sec = default */
-static int	fsync_interval = 0; /* 0 = default */
 static volatile bool time_to_abort = false;
 static bool do_create_slot = false;
 static bool do_drop_slot = false;
+static bool synchronous = false;
 
 
 static void usage(void);
@@ -66,12 +66,11 @@ usage(void)
 	printf(_("  %s [OPTION]...\n"), progname);
 	printf(_("\nOptions:\n"));
 	printf(_("  -D, --directory=DIR    receive transaction log files into this directory\n"));
-	printf(_("  -F  --fsync-interval=SECS\n"
-			 "                         time between fsyncs to transaction log files (default: %d)\n"), (fsync_interval / 1000));
 	printf(_("  -n, --no-loop          do not loop on connection lost\n"));
 	printf(_("  -s, --status-interval=SECS\n"
 			 "                         time between status packets sent to server (default: %d)\n"), (standby_message_timeout / 1000));
 	printf(_("  -S, --slot=SLOTNAME    replication slot to use\n"));
+	printf(_("      --synchronous      flush transaction log immediately after writing\n"));
 	printf(_("  -v, --verbose          output verbose messages\n"));
 	printf(_("  -V, --version          output version information, then exit\n"));
 	printf(_("  -?, --help             show this help, then exit\n"));
@@ -343,7 +342,7 @@ StreamLog(void)
 
 	ReceiveXlogStream(conn, startpos, starttli, NULL, basedir,
 					  stop_streaming, standby_message_timeout, ".partial",
-					  fsync_interval);
+					  synchronous);
 
 	PQfinish(conn);
 	conn = NULL;
@@ -374,7 +373,6 @@ main(int argc, char **argv)
 		{"port", required_argument, NULL, 'p'},
 		{"username", required_argument, NULL, 'U'},
 		{"no-loop", no_argument, NULL, 'n'},
-		{"fsync-interval", required_argument, NULL, 'F'},
 		{"no-password", no_argument, NULL, 'w'},
 		{"password", no_argument, NULL, 'W'},
 		{"status-interval", required_argument, NULL, 's'},
@@ -383,6 +381,7 @@ main(int argc, char **argv)
 /* action */
 		{"create-slot", no_argument, NULL, 1},
 		{"drop-slot", no_argument, NULL, 2},
+		{"synchronous", no_argument, NULL, 3},
 		{NULL, 0, NULL, 0}
 	};
 
@@ -408,7 +407,7 @@ main(int argc, char **argv)
 		}
 	}
 
-	while ((c = getopt_long(argc, argv, "D:d:h:p:U:s:S:nF:wWv",
+	while ((c = getopt_long(argc, argv, "D:d:h:p:U:s:S:nwWv",
 							long_options, &option_index)) != -1)
 	{
 		switch (c)
@@ -455,15 +454,6 @@ main(int argc, char **argv)
 			case 'n':
 				noloop = 1;
 				break;
-		case 'F':
-			fsync_interval = atoi(optarg) * 1000;
-			if (fsync_interval < -1000)
-			{
-				fprintf(stderr, _("%s: invalid fsync interval \"%s\"\n"),
-						progname, optarg);
-				exit(1);
-			}
-			break;
 			case 'v':
 				verbose++;
 				break;
@@ -474,6 +464,9 @@ main(int argc, char **argv)
 			case 2:
 				do_drop_slot = true;
 				break;
+			case 3:
+				synchronous = true;
+				break;
 			default:
 
 				/*

src/bin/pg_basebackup/receivelog.c

@@ -31,14 +31,13 @@ static char current_walfile_name[MAXPGPATH] = "";
 static bool reportFlushPosition = false;
 static XLogRecPtr lastFlushPosition = InvalidXLogRecPtr;
 
-static int64 last_fsync = -1;		/* timestamp of last WAL file flush */
 static bool still_sending = true;		/* feedback still needs to be sent? */
 
 static PGresult *HandleCopyStream(PGconn *conn, XLogRecPtr startpos,
 				 uint32 timeline, char *basedir,
 			   stream_stop_callback stream_stop, int standby_message_timeout,
 				  char *partial_suffix, XLogRecPtr *stoppos,
-				  int fsync_interval);
+				  bool synchronous);
 static int CopyStreamPoll(PGconn *conn, long timeout_ms);
 static int CopyStreamReceive(PGconn *conn, long timeout, char **buffer);
 static bool ProcessKeepaliveMsg(PGconn *conn, char *copybuf, int len,
@@ -55,8 +54,7 @@ static bool CheckCopyStreamStop(PGconn *conn, XLogRecPtr blockpos,
 								stream_stop_callback stream_stop,
 								char *partial_suffix, XLogRecPtr *stoppos);
 static long CalculateCopyStreamSleeptime(int64 now, int standby_message_timeout,
-										 int64 last_status, int fsync_interval,
-										 XLogRecPtr blockpos);
+										 int64 last_status);
 
 static bool ReadEndOfStreamingResult(PGresult *res, XLogRecPtr *startpos,
 						 uint32 *timeline);
@@ -209,7 +207,6 @@ close_walfile(char *basedir, char *partial_suffix, XLogRecPtr pos)
 				progname, current_walfile_name, partial_suffix);
 
 	lastFlushPosition = pos;
-	last_fsync = feGetCurrentTimestamp();
 	return true;
 }
 
@@ -440,8 +437,8 @@ CheckServerVersionForStreaming(PGconn *conn)
  * allows you to tell the difference between partial and completed files,
  * so that you can continue later where you left.
  *
- * fsync_interval controls how often we flush to the received WAL file,
- * in milliseconds.
+ * If 'synchronous' is true, the received WAL is flushed as soon as written,
+ * otherwise only when the WAL file is closed.
  *
  * Note: The log position *must* be at a log segment start!
  */
@@ -450,7 +447,7 @@ ReceiveXlogStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 				  char *sysidentifier, char *basedir,
 				  stream_stop_callback stream_stop,
 				  int standby_message_timeout, char *partial_suffix,
-				  int fsync_interval)
+				  bool synchronous)
 {
 	char		query[128];
 	char		slotcmd[128];
@@ -595,7 +592,7 @@ ReceiveXlogStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 		/* Stream the WAL */
 		res = HandleCopyStream(conn, startpos, timeline, basedir, stream_stop,
 							   standby_message_timeout, partial_suffix,
-							   &stoppos, fsync_interval);
+							   &stoppos, synchronous);
 		if (res == NULL)
 			goto error;
 
@@ -760,7 +757,7 @@ static PGresult *
 HandleCopyStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 				 char *basedir, stream_stop_callback stream_stop,
 				 int standby_message_timeout, char *partial_suffix,
-				 XLogRecPtr *stoppos, int fsync_interval)
+				 XLogRecPtr *stoppos, bool synchronous)
 {
 	char	   *copybuf = NULL;
 	int64		last_status = -1;
@@ -784,14 +781,10 @@ HandleCopyStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 		now = feGetCurrentTimestamp();
 
 		/*
-		 * If fsync_interval has elapsed since last WAL flush and we've written
-		 * some WAL data, flush them to disk.
+		 * If synchronous option is true, issue sync command as soon as
+		 * there are WAL data which has not been flushed yet.
 		 */
-		if (lastFlushPosition < blockpos &&
-			walfile != -1 &&
-			((fsync_interval > 0 &&
-			  feTimestampDifferenceExceeds(last_fsync, now, fsync_interval)) ||
-			 fsync_interval < 0))
+		if (synchronous && lastFlushPosition < blockpos && walfile != -1)
 		{
 			if (fsync(walfile) != 0)
 			{
@@ -799,9 +792,15 @@ HandleCopyStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 						progname, current_walfile_name, strerror(errno));
 				goto error;
 			}
-
 			lastFlushPosition = blockpos;
-			last_fsync = now;
+
+			/*
+			 * Send feedback so that the server sees the latest WAL locations
+			 * immediately.
+			 */
+			if (!sendFeedback(conn, blockpos, now, false))
+				goto error;
+			last_status = now;
 		}
 
 		/*
@@ -821,7 +820,7 @@ HandleCopyStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 		 * Calculate how long send/receive loops should sleep
 		 */
 		sleeptime = CalculateCopyStreamSleeptime(now, standby_message_timeout,
-												 last_status, fsync_interval, blockpos);
+												 last_status);
 
 		r = CopyStreamReceive(conn, sleeptime, &copybuf);
 		while (r != 0)
@@ -1244,34 +1243,22 @@ CheckCopyStreamStop(PGconn *conn, XLogRecPtr blockpos, uint32 timeline,
  */
 static long
 CalculateCopyStreamSleeptime(int64 now, int standby_message_timeout,
-							 int64 last_status, int fsync_interval, XLogRecPtr blockpos)
+							 int64 last_status)
 {
-	int64		targettime = 0;
 	int64		status_targettime = 0;
-	int64		fsync_targettime = 0;
 	long		sleeptime;
 
 	if (standby_message_timeout && still_sending)
 		status_targettime = last_status +
 			(standby_message_timeout - 1) * ((int64) 1000);
 
-	if (fsync_interval > 0 && lastFlushPosition < blockpos)
-		fsync_targettime = last_fsync +
-			(fsync_interval - 1) * ((int64) 1000);
-
-	if ((status_targettime < fsync_targettime && status_targettime > 0) ||
-		fsync_targettime == 0)
-		targettime = status_targettime;
-	else
-		targettime = fsync_targettime;
-
-	if (targettime > 0)
+	if (status_targettime > 0)
 	{
 		long		secs;
 		int			usecs;
 
 		feTimestampDifference(now,
-							  targettime,
+							  status_targettime,
 							  &secs,
 							  &usecs);
 		/* Always sleep at least 1 sec */

src/bin/pg_basebackup/receivelog.h

@@ -31,6 +31,6 @@ extern bool ReceiveXlogStream(PGconn *conn,
 				  stream_stop_callback stream_stop,
 				  int standby_message_timeout,
 				  char *partial_suffix,
-				  int fsync_interval);
+				  bool synchronous);
 
 #endif	/* RECEIVELOG_H */